Chromium plating



Dec. 18, 1962 A. J. DEYRUP CHROMIUM PLATING Filed July 25, 1961 FIG.

FIG.2

INVENTOR DEYRUP ALDEN J.

ATTORNEY 3,069,333 Patented Dec. 18, 1952 3,069,333 CHROMIUM PLATING Alden J. Deyrup, West Chester, Pa., assignor to E. I. du Pont de Nemours and Company, Wilmington, Del., a corporation of Delaware Filed July 25, 1961, Ser. No. 126,623 17 Claims. (Cl. 204-51) This invention relates to chromium plating, and more particularly it relates to chromium plating from a new and improved electrolytic plating bath. 7 This application is a continuation-in-part of my copending application Serial No. 33,951, filed June 6, 1960, now abandoned, which in turn is a continuation-in-part of application Serial No. 844,875, filed October 7, 1959, now abandoned.

The present commercial chromium plating processes are based on the electrolysis of chromium trioxide, CrO (chromic acid), solutions containing small amounts of a catalyst, e.g., sulfates, fluorides or the like. In such commercial processes, for the production of bright plates of acceptable quality the current density and temperature during plating must be closely controlled. Even when closely controlling the current density, temperature and chromic acid-catalyst ratio, the throwing power of the plating bath is very low as compared to other metal plating processes. Because of poor throwing power, it is necessary to provide anodes conforming to the shape of the object to be plated. Moreover, the current efliciency of the commercial plating baths are usually no greater than 8-12% and under the most optimum conditions only 15-20%.

Also, objectionably large volumes of oxygen and hydrogen are given off during plating as a result of which a highly noxious and corrosive spray of chromic acid is always present over the plating bath during the plating operation. Again, large amounts of chromic acid are lost by drag-out and spray due to the necessity of employing rather highly concentrated amounts of chromic acid in commercial baths. The art also discloses chromium plating processes employing trivalent chromium salts. Such previously known trivalent chromium plating processes have had no commercial significance.

It is an object of this invention to provide a new and improved electrolytic chromium plating bath.

It is'another object to provide electrolytic chromium plating baths that have relatively good throwing power, and improved current efliciency.

It is another object to 'provide a new and improved electrolytic chromium plating bath in which the chromium is plated from a divalent chromium salt.

It is yet another object to provide a new and improved electrolytic chromium plating process that will exhibit improved current efficiency and throwing power and will produce an acceptable bright chromium plate.

It is a further object of this invention to provide a new and improved electrolytic chromium plating process that will deposit a new and different form of chromium having an exceedingly high tensile strength.

It is yet another object to produce a self-sustaining chromium foil having a high tensile strength.

Other objects of the invention will become apparent by reference to the following description and claims.

The objects of this invention may be accomplished, in general, by preparing an electrolytic aqueous chromium plating bath comprising a chromium salt in which the chromium has the valence of 2 (he einafter referred to as a divalent chromium salt). at least one carboxylic acid taken from the roup consisting of formic and glycolic acids, and an alkali metal salt taken from the group consisting of sodium and potassium formates and glycolates,

and sodium and potassium salts of a strong acid, i.e., an

acid having a dissociation constant greater than 10- The dissociation constant K is defined as where (H (A) and (HA) are respectively the molar concentrations of hydrogen ion, acid anion and undissociated acid. Many of the strong acids such as HCl and H have dissociation constants too large to measure, much greater than 10*, so they ordinarily do not appear in published tables of dissociation constants. The bath may also advantageously contain boric acid, and, if boric acid is present, it may also advantageously contain sodium fluoride. Boric acid and/or sodium fluoride are not however essential ingredients of the chromium plating bath of this invention.

This electrolytic plating bath may be operated electrolytically at room temperature, or at elevated temperatures up to C., to deposit bright continuous, highly corrosion-resistant chromium plate. The metallic surface to be plated is first thoroughly cleaned in accordance with cleaning procedures well established in the art. Preferably, the metallic surface to be plated, e.g., a copper or nickel surface, is smooth and polished so as to produce a bright chromium plated finish. The metallic object to be plated is suspended as the cathode in the aforesaid electrolytic bath and spaced fairly evenly from an inert anode, e.g., a carbon or graphite anode. The

plating may be carried out by passing an electric current of 30 to amps/sq. ft. at room temperature up to 200-450 amps/sq. ft. at elevated temperatures above 60 C. between said cathode and anode.

In comparison with the trivalent-chromium plating baths described and claimed in my Patent No. 3,006,823, the present divalent-chromium plating baths have higher current efficiencies, higher maximum plating speeds and produce a whiter chromium deposit. I It has, moreover, been found, in accordance with this invention. that under certain specific conditions as hereinafter set forth, it is possible to produce a new and previously unknown form of chromium having an exceedingly high tensile strength, being completely self-supporting even in the form of a thin electroplated film, having very different electrical resistivity and temperature co etficient of resistivity than previously known chromium and being very useful in the form of films or foils.

Such unique form of chromium may be obtained by the electrolytic deposition from the above-described chromium plating bath when the sole carboxylic acid present is formic acid, the sole carboxylate is sodium or potassium formate, the plating temperature is above 50 C. and the bath contains less than 10 parts per million of either sulfur or selenium, impurities normally found in chro-' mium. The accompanying illustrations are drawings of X-ray diffraction patterns of chromium electrodeposits, in which: FIGURE 1 illustrated the X-ray diffraction pattern of chromium produced by the conventional commercial hexavalent chromium plating process. and FIGURE 2 illustrates the X-ray diffraction pattern of chromium produced by the divalent chromium plating process of this invention.

Referring to the drawings, the sharp distinction between successive X-ray diffraction bands in FIGURE 1 clearly indicates that the chromium deposited from the prior art hexavalent process has a distinct crystallin ty of large well-defined crystals. The wash d-out indistinct X-ray diffraction bands shown in FIGURE 2 designate a lack of d stinct, well-defined crystallinity or crystals of an exceedingly small size.

The plating baths of this invention are greatly superior to commercial hexavalent chromium plating baths'inaoeasse current efficiency, throwing power and plating range. These terms are, for convenience, defined as follows:

Current efficiency" is generally defined as the ratio of weight of metal actually deposited by a given current for a given time to that which would have been deposited if electro-reduction of'the desired metal were 100% eflicient. The latter is calculated from Faradays law, which say that 96,500 coulombs (ampere-seconds) is required to electrodeposit one gram equivalent of metal, if no sidereactions occur. such as liberation of hydrogen. Current efliciency is commonly expressed as percent. It is important to note that current efiiciencies as thus calculated are not directly comparable between VI, III, and II-valent chromium. Thus, the best that could possibly be deposited by 96,500 coulombs from hexavalent, trivalent, and divalent chromium salts are, respectively, 52+6, 52+3, and 52:-2 grams. Thus, a current efficiency of 15% with a trivalent bath is just as good as 30% with a hexavalent bath, or only as good as with a divalent bath. Current efliciency is important because it governs in part the rate of plating, and so is of great economic importance. However, the cost of current is only one factor in the economics of plating. Probably of more practicalimportance is the maximum speed of plating, using as much current as will be tolerated without appearance of defects discussed below under plating range.

Throwing power is a term used in, the plating industry to signify the degree to which plating occurs in recesses or on surfaces which are remote from the anode as compared with plating on flat surfaces or those near to the anode. Every one in the plating industry knows what good throwing power is, and that commercial chromium plating baths have very poor throwing power. However, there does not appear to be a generally accepted measure of throwing power as such. To a principal degree, the differences in plating on the variously disposed. surfaces of an article in a plating bath are governedby the response of the plating system to the local current densities. Therefore, we can secure good information about throwing power by measuring plating range.

It is frequently true of electroplating baths that below certain current densities (amperes per unit area) they either do not deposit metal at all, or if they deposit metal, itiis defective in brightness, color, or other respect. Above an upper limiting current density characteristic of each specific bath the plating is burned or discolored, matt, spongy, or otherwise defective. Between these limits, which I will call threshold current density and ceiling current density, good plating occurs. It is evident that if this plating range is very narrow, the system will have poor throwing power. This is a present handicap in. commercial chromium plating. It is also evident that if this range is very broad and the ceiling current density occurs at very heavy currents, then plating can be very rapid, even ifcurrent efficiency is only moderately good.

The divalent chromium salt may be composed of any chromous compound which will dissolve in the plating bath and which will not introduce into the said bath nonoxidizing anions. For example, chromous chloride, CrCl chromous bromide, CrBr chromous iodide, Crl chromius fluoride, CrF chromous for-mate,

chromous glycolate, Cr(CH OH-CO -H O; or chromous sulfamate, Cr(SO NH may be used for this purpose.

The chromous salts of acids having an ionization constant greater than 10- are preferred, however, a few chromous salt of acids having an ionization constant less than 10- such as chromous carbonate may also be used. Except for chromous formate and/or chromous glycolate, chromous salts of acids having ionization constants between 10 and 10- are much less desirable because of lowrplating efiiciency. When it .is desired to use a bath containing a fluoride, it has been found most expedient to use sodium fluochromate (II), Na CrF or when it is desired to use a bath containing boron but not containing the fluoride, it is advantageous to use a compound, CrOI-IX-I-I BO (X=Cl, Br, or I). These compounds are air-stable salts, whereas many chromous salts are not air stable and, therefore, must be protected from the atmosphere during shipping and handling.

The preparation ofNa CrF is described'and claimed in my US. Patent No. 2,996,353. The preparation of compounds of the formula CrOHX-H BO is described in my copending application, Serial No. 118,788, filed June 22, 1961.

The carboxylic acid present in the plating bath must be formic acid or glycoiic acid or a mixture thereof. It may be added either as the acid itself or as an alkali metal formate or glycolate together with a strong acid such as hydrochloric acid (an acid having a dissociation constant of at least 10* which will react to produce the desired formic or glycolic acid in situ, care being taken to avoid an excess of the strong acid which might reduce the pH below about 1.7.

In general, an alkali metal formate, glycolate, or mixture thereof may be present as the sodium or potassium salt of either of these acids. The alkali metal carboxylate need not be derived from the same carboxylic acid as the carboxylic acid used in making the bath. For example, the carboxylic acid may be formic acid, and the alkali metal carboxylate may be added as sodium glycolate. It is not necessary in all cases to have an alkali metal formate or glycolate present in the oath, although sometimes it is preferable. If the bath contains no alkali metal carboxylate, then it is essential to have present in the bath an alkali salt of a strong acid (dissociation constant l0- as, for example, sodium chloride.

In cases where fluoride is used in the bath, it should be sodium fluoride. In the absence of fluoride, alkali salts" may be present, and are usually desirable to improve conductivity, and these may be either sodium or potassium or, in some cases, ammonium salts. However, when fluorides are used in the bath, potassium and ammonium salts from any source are undesirable because they may remove both fluorine and boron as insoluble fluoborates.

Boric acid, when used, may be added as borax, boron oxide, boric acid, or sodium oxyfluoborate, a complex compound having the formula 4NaF-5B O (see US. Patent No. 2,823,095). All of these materials are equivalent to boric acid in aqueous solution and react toform the desired BF OH ion further discussed below. Although fluoborates, i.e., the well-known compounds containing the BF; ion, may be used as the source of boron, they are not recommended since the baths are not satisfactory for a period of days until the fluoborates have had time to hydrolyze to make the desired complexes.

The essential components of the plating bath are (1) a divalent chromium salt, (2) formic acid or glycolic acid or mixtures thereof, and (3) either an alkali formate or glycolate or mixture thereof, or an alkali salt of a strong acid (dissociation constant 10- In addition to these essential components, desirable components for some purposes are boric acid or boric acid and sodium fluoride. The above-described essential and nonessential but desirable components of the plating bath may be constituted of compounds which may comprise one or more of these components. For example, sodium oxyfluoborate may be the source of both the fluoride and the boric acid; or the sodium fiuochromate may be the source of both the essential divalent chromium salt and'the fluoride; or chromium formate may be the source of both the divalent chromium and the formate radical. The es sential constituents of the electroplating bath of this invention are preferentially present in the plating bath in certain proportions. This is also true of the desirable but nonessential constituents. In 1000 grams of plating solution, it is preferred that the several constituents be Divalent chromium salt 0.1 to 2 Free formic and/or glycolic acid 0.3 to 1 Alkali formate and/ or glycolate, or

alkali metal salt of strong acid 0.3 to 4 Fluoride (when boric acid is present) to Boric acid 0 to 3 Within the concentration ranges specified, it should be noted that the more dilute solutions may have lower current efificiencies and/or lower maximum plating ranges. To counteract this disadvantage of low concentrations is the advantage that less chemicals are lost by drag-out of plating solution. At the upper end of the concentration range, we generally do not lose in current efliciency or plating rate or character of deposit. Rather, we are likely to run into solubility limits of the various reacting species in the solution so that precipitation occurs. This does not, of itself, impair plating, but precipitated ingredients obviously are of no utility. Another obstacle at high concentrations is increasing viscosity of the solution, and decreasing conductivity, with correspondingly increased voltage and power consumption. Some of the compositions given in Table 1, below, may precipitate crystalline material at room temperature. This does not ordinarily interfere with their useful plating. However, the precipitated material should be left in if one chooses to plate at an elevated temperature, when it will redissolve.

The function of the boric acid in a plating bath of this invention not containing a fluoride is to act directly as a buffer to control pH at the cathode. The presence of boric acid usually broadens the conditions of temperature and current density at which plating is satisfactory. The presence of boric acid increases the current density at which bright chromium can be plated without decreasing the threshold current density at which chromium first deposits. When boric acid is used, and no fluoride is present, the pH should be kept within the range of 1.7 to 3.5. If it is desired to operate a chromium plating bath of this invention at a pH of 27, then fluoride should be excluded from the bath because fluoride in the presence of boric acid at pHs lower than 2.7 increasingly reacts to form lluoborate ion, 3P4", which I find to be useless or even harmful in the system. When boric acid and sodium fluoride are present in the bath together, the pH may be used in the range 2.7 to 4.5.

Under some circumstances it may be desired to plate chromium in accordance with this invention without the presence of fluorine, as noted below. However, when it is desired to attain the broadest working range of current densities and also highest plating rates, then the system containing both boric acid and fluoride is preferred. My experiments indicate that fluoride and boric acid in these plating baths in the pH range of 2.7 to 4.5 react rapidly and almost completely to form the ion 31 01 1. In the absence of boric acid, sodium fluoride in this pH range becomes sodium bifluoride by the reactioni If boric acid is present, however, the following reaction occurs very rapidly:

H++3HF +2H BO -2BF OH+4H O The BF OH" ion acts as a powerful buffer capable of destroying iydroxyl ions near the cathode by a reaction such as:

BF OH-+2OH+H BO +3F* When boron and fluorine compounds are placed together in water solution, the useful BF OH- ion is present and stable in the pH range of 2.7 to 4.5. At higher pHs this becomes unstable relative to bifluoride ion, H1 and free boric acid. At pHs below 2.7 the BF OH- ion becomes unstable relative to the well-known fluoborate ion, BF4 These pH limits may not be absolutely sharp, but approximately define the limits of equilibrium stability of the BF OH- ion.

In choosing the amounts of fluorine and boron compounds to be used in preparing my fluorine-containing baths it is desirable to use enough boric acid or equivalent not only to form the BF OH ion but also to practically saturate the bath with additional boric acid. This procedure tends to stabilize the fluorine as BP OH-. If, on

the other hand, an excess of fluoride is used, relative to boric acid, then the excess tends to form a bifluoride such as Naf-IP which is only sparingly soluble. There is no sharp dividing line, but in general as the BF OH- ion concentration is raised the working range of current densities of the plating bath becomes desirably broader. A. imit on the amount of BF OH- which can practically be used is caused by the fact that bifluoride ions are in The conditions above described are suitable for depo-' sltion of bright chromium resembling in good appearance,

color, and brilliance the chromium conventionally deposited from hexavalent chromiumbaths by present com-. mercial processes. This sort of chromium coating imparts not only good appearance, but also surface hardness. and a modest amount of additional corrosion protection when used for plating hardware and the like. However, in general, bright chromium deposits of whatever sort have heretofore been usually cracked and always brittle. The literature gives ways of avoiding cracking and improving the over-all corrosion resistance thereby. Even when cracking is avoided, however, all previously-known e'lectrodeposits, including some of those produced in accordance with this invention, are brittle or weak or both. The brittleness and/or weakness is not apparent when chromium is applied as a coating on other metals. However, when the cromium layer is removed, as by dissolving away from it a temporary base metal, the chromium layer is always found to disintegrate into tiny flakes because of inherent brittleness and weakness. I have found within the broad range of conditions of this invention, all of which are suitable for bright decorative plating of metal articles, a narrower range of conditions under which one may deposit chromium which has an exceedingly high tensile strength and under some conditions has a high degree of plastic deformability. These narrowed conditions comprise the following essentials; (1) formic acid with or without formate as the sole carboxylates; (2) a deposition temperature above about 50 C.; (3) substantially complete freedom from sulfur and selenium compounds (less than 10 ppm). When these narrowed conditions are used, it has been noted that plated hardware becomes more resistant to corrosion, after deformation thereof, thanwhen previously-known chromium plating is used. Also, by depositing such chromium films on a temporary metal base, such as brass, it is possible to release sheets of chromium foil when the base metal is dissolved away, as by use ofnitric acid. These sheets of foil are in marked contrast to the minute flakes of weak, brittle chromium deposited by other ways, even though the latter may be quite satisfactory for decorative plate. These chromium foils have been found to possess some ductility at thicknesses less than 50x10 inches. At thicknesses greater than 50 10- inches, little or no ductility remains, but strength measurements indicate the same high tensile strengths as the thinner films. The tensile strength of chromium electrodeposited by the above-said narrow ranges of conditions has been found to vary between 200,000 to 450,000 lbs. per square inch by actual test on weighted strips. The highest previously reported tensile strength of chromium is under 114,000 lbs. per square inch. Such thin, ductile and/ or high-strength chromium metal films should be of utility where high film strength and exceptional corrosion resistance are desired in metal foils. For example, they may be used advantageously for the metal transfer films of my copending patent application, Serial No. 33,951, filed June 6, 1960. While the ductility and strength of such foils as deposited and released are exceptional for chromium, it is found that these properties can be enhanced by baking the chromium films either before or after release at temperatures from 200 to 350 C. for periods of a few seconds up to one hour. The high tensile strength chromium as produced within the aforesaid narrow ranges of conditions also dififers from previously known chromium in its electrical properties. Whereas prior art chromium has an electrical resistivity of 13x10 ohm cm., this chromium has a resistivity of l49i9 10" ohm cm. Moreover, this chromium has a much lower temperature coeflicient of resistivity than prior art chromium, i.e., :5 l0 per degree C. versus about 2,000 to 3,000 l0 per degree C.

With, regard to the purity of the chromium deposition bath, I have found that as little as 10 pp m. of either sulfur or seleniumin the electrolytic plating bath is quite detrimental to, the physical strength of the resultant chromium foil. Fortunately, it is relative'y easy to eliminate. compounds of these elementsfrom my bath by boiling and evaporation, It appears that most sulfur and selenium compounds are reduced by the divalent chromium ion to hydrogen selenide and hydrogen sulfide, which gases are removed with the steam during evaporation. Ordinarily, no special treatment is needed to remove these impurities from my baths, beyond a partial evaporation of the water therein. However, some of the formic acid may also be lost in this evaporation and should be replaced as indicated by analysis of the solution. Even when these impurities were ccmpletely removed from a bath containing glycolfc acid, a ductile or strong chromium foil dd not result even at a deposition temperature of 58 C. Also, by way of comparison, a thoroughly purified solution containing formic acid as the only carboxylic acid did not yield a ductile or strong chromium foil when deposition took place at 25 C.; the same solution yielded both ductile and high-strength chromium foils when the electrodeposition is carried out between 50 and 90 C.

Divalent chromium compounds in solution are oxidized by exposure to air. Hence it is desirable to pad the electrolytic cell with an inert gas such as nitrogen. Another workable process is to keep the bath substantially covered with an inert foam. Such a foam wil often form on the bath during use. It may also be desirable to store the previously prepared baths under an inert gas, e.g., nitrogen to minimize air-oxidation of the divalent chromium. A little chromium oxidation does no great harm. The

trivalent chromium salts formed by oxidation do not appear to poison or harm the bath. However, if a large proportion of the divalent chromium becomes oxidized to trivalent chromium, the bath stops plating. The amount of chromous ion in the bath must be sufficient that the chromium metal plated out is from the chromous ionrather than the chromic ion.

It may be found advantageous to improve the electric conductivity of the bath by the addition of alkali salts of st'ong acids (dissociation constant l0 such as sodium chloride, bromide or iodide. The foreign anions such as chloride ion may cause some problems at the anode. This may be avoided by placing the anode in a separate compartment with a porous diaphragm to limit mixing of the solution from the anode and cathode regions, or they may be completely avoided by use of a cation permeable membrane. Cloths coated with cation exch nge resins are commercially used in preventing transfer of anions while permitting free passage of cations.

Salts of weak acids (dissociation constant l0- should not be added except those of essential formic and/ or g ycolic acids. Such sats of weak acids react with the formic and/or glycolic acids, removing these essential materials. For example, acetic acid (dissociation constant about 10 and acetates are detrimental, incre:singly so with increasing concentration. It is also to be noted that alkali salts should not be added of those acids whose anions might be reduced at the cathode, such as nitrates.

Surf. ce-active agents may be added to the plating baths of this invention. Such agents have several useful purposes. For example, Petowet R, a satura ed hydrocarbon sodium sulfonate, is advantageously added in amounts of the order of 0.02% to reduce bubble size and improve uniformity of deposition and hence maximum platjng speed, i.e., the maximum rate of deposition of bright metal in units of Weight per unit area per unit time. Normal octyl alcohol in amounts of the order of 0.01% may be added to counteract the excessive foaming tendency of Petrowet R and also to reduce any tenden:y of pitting.

Within the range of pH 1.7 to 4.5, the higher the pH the lower the threshold current density (other conditions being held con:tant) but also the lower the ceiling current density of bright plating. The former is a desirable result but the latter is undesirable. Increasing quantities of tin-ionized cids at constant pH has to some extent EXAMPLE I An electrolytic plating bath was made with the following substances in gram-moles per 1,000 grams of solution:

Chromium (ll) chloride, CrCl t 0.4 Formic acid, I-I CO 0.5

Sodium chloride, NaCl 0.5,

This was electrclyzed at about 55 C. in a cell with a diaphragm, to separate anode and cathode compartments. A platinum anode and a polished brass cathode were used. At a current density of 340 amp/sq. ft. a very bright chromium deposit was obtained at a rate of about 65 mic. o-inches per minute and current efficiency of about 20%. When a specimen of the chromium deposit was recovered as foil, by solution of the brass plate in niric acid, it was found to be 43 microinches in thickness. The foil was exceptionally strong, considering its thinness. When folded double and creased, 'it did not break in the usual manner of brittle chromium, but retained the creased form.

EXAMPLE H One gram-mole of chromium (ll) chloride in solution was prepared by reacting at -l00 C. an excess of commercial pure chromium with two gram-moles of hydrogen chloride in a total volume of 510 ml. of water, the acid being slowly added to avoid excessive reaction speed. The solution was filtered to remove excess chromium metal and evaporated in vacuo to dryness, in order to free it of traces of volatile selenium and sulfur compounds. The chromium (ll) chloride was dissolved in water; 0.2 gram-mole of sodium formate and 0.6 gram-mole of formic acid were added; and the solution was made up with water to a weight of 1,000 gm. These operations were conducted under a nitrogen atmosphere. The solution was used for electrodeposition of chromium in the manner set forth in Example I at various temperatures from 25 C to 75 C. and at various current densities from 50 to 420 amp/sq. ft., resulting in very bright chromium deposits, at each temperature over a useful range of current densities. As one instance, electrolysis at 5560 C. with 280 amp. per sq. ft. yielded an extremely bright plate on a brass substrate, at a rate of 32 microinches/min. and current emciency 9 of 11%. The plating was released from the brass substrate, by solution of the latter with a 20% nitric acid solution, as a single sheet of bright foil, with exceptionally high strength.

- 10 tests showed results substantially identical to those of Example III.

EXAMPLE )GX Electrolytic plating baths identical to that of Example EXAMPLE HI Ill were prepared with the sole exception that CrCl was 0.57 gram-mole of purified chromium (ll) chloride replaced respectively with (a) Cr(HCO -2H O, (b)

was prepared as in Example 11. To this was added 1.17 Cr(CH OH-CO 'H 0, (c) CrBr and (d) CrI In gram -moles sodium formate, 0.57 gram mole formic each case plating results with these modified baths were acid, 1.33 gram-moles boric acid, 2.90 gram-moles of 50- substantially the same as the results set forth in Example dium chloride, and water to a total weight of 1,000 gm. ill. To this solution was added also 0.15% by weight of The electrolytic plating baths of this invention may Petrowet R, a saturated hydrocarbon sodium sulfocontain stabilization agents, wetting agents, oxidation innate and 0.1% by weight of normal octyl alcohol, these hibitors or other additive materials commonly used in additions being convenient for improving uniformity of the plating art, inasmuch as the plating baths herein distribution of electrodeposits. This solution, when elec- 15 disclosed have a tendency toward oxidation, both from trodeposited, yielded very bright electrodeposits over a the air and at the anode as a result of oxygen formawide range of temperatures from 7 to 90 C. and curtion at the anode, it is desirable to maintain the bath rent densities from 85 to over 450 amps/sq. ft., the useclosed to the atmosphere or to surround or shield the ful range of current densities increasing With temperaanode with a layer of paper or other ion-permeable ture. fil

As exemplary of the effect of deposition temperature The plating baths of this invention and the process n th p ysical prope ties f the r sultant Chr mium, of plating therewith has produced bright chromium electhere are described below electrodeposits A, B and C. trodeposits at current densities of 20 to 450 amps. per sq. All of these were deposited to the same thickness on ft. The throwing power of the baths is generally greater brass panels from a bath of the above composition, but than that of commercial hexavalent chromium plating respectively at 6 C., 25 C., and 55 C. All the l baths and current efficiencies of 15 to 40%, based on trodeposited panels were baked at 300 C. for 15 mi divalent chromium are obtainable. Comparable current Utes, and Lhfih Placed in 121 nitric acid-Water mixture efficiencies of conventional commercial hexavalent chrote dissolve the brass' In cases A and the m m minm plating baths would have to be 45% to 120%, the was so weak and brittle that it spontaneously disint latter figure being, of course, impossible, whereas such grated into 51112111 fiakes- In Case however, an P- commercial processes usually have efficiencies no greater tionally strong foil resulted having a thickness of about than 842% and at most 20%.

125 microinches. Direct tensile strength measurements Si it i b i h many changes d iii were made 011 ribbons P p from this fOiL The tions can be made in the above-described details withtensile strengths recorded on difierent samples f the out departing from the nature and spirit of the invention, Same Were 279,O60, and 450,000 Pounds P it is to be understood that the invention is not to be square hlch- In the Case of the lower figures it is limited to said details except as set forth in the appended lieved that accidental edge flaws made during prepara- 1 i I tion of the test strip were responsible for breakage, and V 1 1 i that the highest figure is closer to the true strength of 40 1 A aqueous electrolytic plating bath f r the plat. is at r a ing of bright chromium plate from chromium in the The following table sets forth the bath ingredients in di l t t id bath having a pH of 1.7 to 4.5 and gram molecular Weiht$ of Examples IV t0 XVII, containing as essential ingredients, per 1,000 g. of bath, clusive. In these examples are also stated the pH value 1 t 2 gram-moles of a di al nt chromium alt, 0.3 n the maXimum rate of bright Plating in microinchfis to 1 gram-mole of at least one carboxylic acid taken from P minute Th6 baths of Examples IX and X contain no the group consisting of formic and glycolic acids and fluOfine- Bath of Example IV contains glycolate ions 0.3 to 4 gram-moles of at least one alkali metal salt but no formate ions whereas the bath of Example V conf an i t k f o th group consisting of formic, thins formate but 110 glycolate ions, and bath of EXamglycolic and acids having a dissociation constant of at ple VI contains both formate and glycolate ions. 'Ihese l t 1 10 were all effective for bright plating in the range Of An aqueous electrolytic plating bath for the platamperes/sq- 's the eptimumewrent y, hOWtiVeI, ing of bright chromium plate from chromium in the tending in all cases to increase with operating temperadivalent state, said bath having a pH of 1.7 to 4.5 and ture. containing as essential ingredients, per 1,000 g. of bath,

Table MOLAR GOMPOSIIIONSDIVALENT FLA-TING BATES Example No IV V VI VII VIII IX X XI XII XIII XIV XV XVI XVII NI12QTF4 0.64 0.03 0.64 0.55 0.26 0.55 1.10 0.55 0.26 0.13

c g 1 0.54 0.51 0.51 0.54 N 2.21 2.21 0.61 0.62 2.82 3.03 4.50 1.10 1.13 0.41 0.16 0.50 1.13 1.34 0.73 0.88 0.68 1.74 1.54 1.03 1.43 1.14 1.01 0.73 0.80 0.68 1.46 0.10 0.80 1.44 1.15 1.02 0.40 1.35 1.10 H3303 .10 3.12 1.34 1.34 3.12 2.34 1.73 3.05 3.05 3.05 3.05 NaOl .15 .72 1.63 2.05 2.04 1.66 3.55 3.00 3.00 3.00 2.41 2.16 Octylalcohol .01 .01 .01 .01 .01 .01 .01 .01 .01 .01 .01 .01 .01 .01 p .18 a. 30 3.08 3.32 3.32 2.48 3.11 3.40 3. 37 3.30 3.70 3.70 3.75 3.37 Dmanmioroiuchlminfln 25 40 40 35 45 55 5 45 50 45 30 10 EXAMPLE XVIII The bath of stoichiometric composition identical to that of Example Ill was prepared, using instead of CrCI the compound CrClOH-H BO amounts of other raw materials were adjusted for this purpose. Electrolytic 0.1 to 2 gram-moles of a divalent chromium salt, 0.3 to l gram-mole of formic acid and 0.3 to 4 gram-moles of at least one alkali metal salt of an acid taken from the group consisting of formic, glycolic and acids having a dissociation constant of at least l 10 3. An aqueous electrolytic plating bath for the plating of bright chromium plate from chromium in the divalent state, said bath having a pH of 1.7 to 4.5 and containing as essential ingredients, per 1,000 g. of bath, 0.1 to 2 gram-moles of a divalent chromium salt, 0.3 to 1 gram-mole of at least one carboxylic acid taken from the group consisting of formic and glycolic acids and 0.3 to 4 gram-moles of an alkali metal formate.

4. An aqueous electrolytic plating bath for the plating of bright chromium plate from chromium in the divalent state, said bath having a pH of 1.7 to 4.5 and containing as essential ingredients, per 1,000 g. of bath, 0.1 to 2 gram-moles of a divalent chromium salt, 0.3 to 1 gram-mole of formic acid and 0.3 to 4 gram-moles of an alkali metal formate.

5. An aqueous electrolytic plating bath for the plating of bright chromium plate from chromium in the divalent state, said bath having a pH of 1.7 to 4.5 and containing as essential ingredients, per 1,000 g. of bath, 0.1 to 2 gram-moles of a divalent chromium salt, 0.3 to l gram-mole of glycolic acid and 0.3 to 4 gram-moles of an alkali metal glycolate.

6. An aqueous electrolytic plating bath for the plating of bright chromium plate from chromium in the divalent state, said bath having a pH of 1.7 to 4.5 and containing as essential ingredients, per 1,000 g. of bath, Oil to 2 gram-moles of a divalent chromium salt, 0.3 to 1 grammole of formic acid'and 0.3 to 4 gram-moles of an alkali metal salt of an acid having a dissociation constant of at least 1 l0 7. An aqueouselectrolytic plating bath for the plating of bright chromium plate from chromium in the divalent state, said bath having a pH of 1.7 to 4.5 and containing as essential ingredients, per 1,000 g. of bath, 0.1 to 2 gram-moles of a diavalent chromium salt, 0.3 to l gram-mole of glycolic acid and 0.3 to 4 gram-moles of an alkali metal salt of an acid having adissociation constant of at least 1X 10" 8. An aqueous electrolytic plating bath for the plating of bright chromium plate from chromium in the divalent state having a tensile strength cf at least 200 000 lbs. per sq. in., said bath having a pH of 1.7 to 4.5 and containing as essential ingfedients, per 1,000 g. of bath, 0.1 to 2 gram-moles of a divalent chromium salt, 0.3 to 1 gram-mole of formic acid as the sole carboxylic acid in the bath and 0.3 to 4 gram-moles of an alkali metal formats as the sole carboxylafe in the bath, said bath containing less than 10 ppm. of a material taken from the group consisting of sulfur and selenium.

9. Chromium electroplate having a tensile strength exceeding 200,000 lbs. per sq. in. and having an electri-cial resistivity of 149i9 l0 ohm cm.

10. The process of electroplating a bright chrrmium electroplate which comprises passing a direct electric current with a current density of 20 to 450 amps/sq. ft. between an inert anode and a metallic catode in an electrolytic plating bath having a pH of 1.7 to 4.5 and containing as essential ingredients, per 1,000 g. of bath, 0.1 to 2 gram-moles of a divalent chromium salt, 03 to 1 gram-mole of at least one carboxylic acid taken from the group consisting of formic and glycolic acids and 0.3 to 4 gram-moles of at least one alkali metal salt of an acid taken from the group consisting of formic, glycolic and acids having a dissociation constant of at least 1 10- 11. The process of electroplating a bright chrmium electroplate which comprises passing a direct electric current with a current density of 20 to 450 amps/sq. ft. between an inert anode and a metallic cathode in an electrolytic plating bath having a pH of 1.7 to 4.5 and containing as essential ingredients, per 1,000 g. of bath, 0.1 to 2 gram-moles of a divalent chromium sat, 0.3 to l gram-mole of formic acid and 0.3 to 4 gram-moles of at least one alkali metal salt of an acid taken from the group consisting of formic, glycolic and acids having a dissociation constant of at least 1x10 12. The process of electrop'ating a bright chrom'um electroplate which comprises passing a direct electric current with a current density of 20 to 450 amps/sq. ft. between an inert anode and a metallic cathcde in an electrolytic plating bath having a pH of 1.7 to 4.5 and containing as essential ingredients, per 1,000 g. of bath, 0.1 to 2 gram-moles of a divalent chromium salt, 0.3 to 1 gram-mole of at least one carboxylic acid taken from the group consisting of formic and glycolic acids and 0.3 to 4 gram-moles of an alkali metal formatc.

13. The process of electroplating a bright chromium electroplate which comprises passing a direct electric current with a current density of 20 to 450 amps/sq. ft. between an inert anode and a metallic cathode in an electrolytic plating bath having a pH of 1.7 to 4.5 and containing as essential ingredients, per 1,000 g. of ta h,

t 0.1 to 2 gram-moles of a divalent chromium salt, 0.3

to l gram-mole of formic acid and,0.3 to 4 gram-moles of an alkali metal formate.

14. The process of electroplating a bright chromium electroplate which comprises passing a direct electric current with a current density of 20 to 450, amps /sq. ft. between an inert anode and ametallic cathode in an electrolytic plating bath having a pH of 1.7 to 4.5 and.

containing as essential ingredients, per 1,000 g. of bath, 0.1 to 2 gram-moles of a divalent chromium salt, 0.3

- to 1 gram-molt of glycolic acid and 0.3 to 4 gram-moles of an alkali metal glycolate.

15. The process of electroplating a bright ch"omium electroplate which comprises passing a direct electric current with a current density. ofv 20 to 450 amps/sq. ft.

between an inert anode and a metallic cathode in anelectrolytic plating bath having a pH of 1.7 to.4.5 and containing as essential ingredients, per 1,000 g. of bath, 0.1 to 2 gram-moles of a divalent chromium salt, 0.3 to 1 gram-mole of formic acid and 0.3 to 4 gram-moles of an alkali metal salt of an acid having a dissociation constant of at least 1 l0- 16. The process of electroplating a bright chromium electroplate which comprises passing a direct electric current with a current density of 20 to 450 amps/sq. ft. between an inert anode and a metallic cathode in an electrolytic plating bath having a pH of 1.7 to 4.5 and containing as essential ingredients, per 1,000 g. of bath,

0.1 to 2 gram-moles of a divalent chromium salt, 0.3

to l gram-mole of glycolic acid and 0.3 to 4 gram-moles of an alkali metal salt of an acid having a dissociation constant of at least 1X10? 17. The process of electrcplating a bright chromium electroplate which comprises passing a direct electric current with a current density of 20 to 450 amps/sq. ft. between an inert anode and a metallic cathode in an electrolytic plating bath having a pH of 1.7 to 4.5 and containing as essential ingredients, per 1,000 g. of bfth, 0.1 to 2 gram-moles of a divalent chromium salt, 0.3 to l gram-mole of formic acid as the sole carboxvlic acid in the bath and 0.3 to 4 gram-moles of an alkali metal formate as the sole carboxylate in the bath, said bath containing less than 10 ppm. of a material taken from the group consisting of sulfur and selenium.

References Qited in the file of this patent UNITED STATES PATENTS 1,922,853 Kissel Aug. 15, 1933 1,975,239 Ungelenk et a1. Oct. 24, 1934 FOREIGN PATENTS 292,094 Great Britain Aug. 8, 1929 

1. AN AQUEOUS ELECTROLYTIC PLATING BATH FOR THE PLATING OF BRIGHT CHROMIUM PLATE FROM CHROMIUM IN THE DIVALENT STATE, SAID BATH HAVING A PH OF 1.7 TO 4.5 AND CONTAINING AS ESSENTIAL INGREDIENTS, PER 1.000 G. OF BATH, 0.1 TO 2 GRAM-MOLES OF A DIVALENT CHROMIUM SALT, 0.3 TO 1 GRAM-MOLE AT LEAST ONE CARBOXYLIC ACID TAKEN FROM THE GROUP CONSISTING OF FORMIC AND GLYCOLIC ACIDS AND 0.3 TO 4 GRAM-MOLES OF AT LEAST ONE ALKALI METAL SALT OF AN ACID TAKEN FROM THE GROUP CONSISTING OF FORMIC, GLYCOLIC AND ACIDS HAVING A DISSOCIATION CONSTANT OF AT LEAST 1X10-2. 